1. Field of the Invention
The present invention generally relates to microwave amplitude modulation systems and, more particularly, to PIN diode modulation systems for modulating the amplitude of microwaves.
2. State of the Art
It is well-known to emploY PIN diode modulators for modulating the amplitude of microwave signals. One of the advantages of PIN diodes in such applications is that those devices, when forward-biased, provide resistance that varies inversely with diode current and, accordingly, provide a voltage-current relationship which is exponential. Another advantage of PIN diodes in microwave modulating applications is that the diodes have generally monotonic frequency response characteristics in the frequency range of microwaves.
For modulating microwaves, one widely used type of PIN diode modulator is the so-called "shunt" type. Typically, shunt-type PIN diode modulators comprise four PIN diodes connected in parallel between a microwave conductor and ground. The video inputs to such shunt-type PIN modulators usually are driven by current sources. Such current drive sources, although providing generally favorable operating characteristics, limit the modulating frequency (as indicated by the modulator's half-power frequency) to the range of about 200 to 500 kilohertz. That modulating range is too narrow for a number of applications in modern, high-speed telecommunications and radar.